Methods are well-known to plate tin over metallic surfaces. The instant baths and methods are to be distinguished from the techniques based upon electrolytic deposition and electroless plating.
Electrolytic plating is the production of adherent deposits of metals on conductive surfaces carried out by passage of electric current through an electroplating solution. The plating rate is determined by the current density impressed on the surface being plated.
Electroless plating is a method of metal deposition without the assistance of an external supply of electrons but, requiring an agent present in the processing solution capable of reducing the ions to be deposited. The process is further characterized by the catalytic nature of the surface which enables the metal to be plated to any thickness. Typically, such solutions comprise a solvent, a supply of ions of the metal to be deposited, an agent capable of reducing the ions of the metal to be deposited, a complexing agent for the ions of the metal to be deposited, and a pH regulator.
Among other problems, a major difficulty is sometimes encountered with depositing electroless metal on closely defined areas. There is a tendency for non-sensitive areas after prolonged immersion in or contact with electroless metal solutions to receive scattered or random metal deposits. In addition, the electroless metal solutions sometimes produce metal deposits which contain a substantial amount of hydrogen causing the deposits to be brittle, breaking under rough mechanical handling and bending.
Immersion plating or "contact plating" depends, however, upon a galvanic displacement reaction. The current instead of being furnished from an outside source, arises from reaction of the substrate itself and the metal being plated. Because of this, metal thickness has traditionally been limited to 10 to 50 millionths of an inch. As the immersion process depends upon the electrolytic action of the base metal, deposition stops as soon as the base metal is entirely covered forming a very thin deposit.